Many analysis applications, such as gas chromatography, require a regulated pressure of a fluid in order to obtain accurate and repeatable measurements. For example, when preparing to deliver a gaseous sample into an analysis column of a gas chromatograph, the sample is first collected in what is referred to as a “sample loop,” and then, under the control of a gas sampling valve, directed (also referred to as “injected”) into the analysis column of the chromatograph. When loading the sample fluid in the sample loop, the amount of a gaseous sample is dependent upon many factors, one of which is the pressure of the fluid in the sample loop. Changes in the ambient pressure (also referred to as barometric pressure) affect the amount of sample molecules contained in the sample volume, resulting in variability of the absolute amount of compounds detected by the chromatographic analysis.
Further, because the sample loop is typically vented to ambient pressure, the absolute amount of molecules contained in the sample loop is less than if the sample loop were at an elevated pressure. This results in a smaller amount of molecules being injected into the chromatographic column for analysis, thus limiting the accuracy of the analysis. Further, because the sample loop is at a relatively low gauge pressure compared to the inlet pressure of the chromatograph into which the sample is injected, there is a resulting pressure and flow disturbance when the contents of the sample loop are injected into chromatograph. This pressure and flow disturbance further reduces the accuracy of the chromatographic analysis.
Prior solutions include the implementation of a mechanical absolute back pressure regulator and pressure accumulator, in which the mechanical pressure regulator becomes the reference for all flows within the chromatograph. However, drawbacks of such a system include the requirement that the mechanical back pressure regulator be frequently calibrated off-line from the system, a lack of programmability and adjustability. Also, because all flows in such a system are referenced to a mechanical pressure regulator, and because mechanical pressure regulators are sensitive to flow, variations in the flow through the regulator, which affect the actual pressure in the system, can manifest as noise on the analysis output, further reducing the accuracy of the analysis.
Further, because all flows in such a system travel through the absolute pressure regulator, any sample components that become chemically active when ionized (e.g., by a flame ionization detector) will likely react and corrode portions of the regulator, thus reducing regulator service life. Further, as the regulator ages due to mechanical drift or as a result of the chemicals passing through the regulator, the results of the analysis will become less reliable.
Accordingly, a need exists for a back pressure regulator in a chromatographic analysis system that overcomes the above-mentioned shortcomings.